1. Field of the Invention
The present invention relates to a method of manufacturing a thermal head for use in thermal recording in a facsimile machine, a printer, or the like.
2. Background Information
Conventionally, as shown in FIGS. 2(a) and (b), a glaze layer 2 as a heat storage layer is provided on an insulating substrate 1 such as a ceramic substrate, a heater resistor material of a Ta system, a silicide system, an Nixe2x80x94Cr system, or the like and an electrode material of Al, Crxe2x80x94Cu, Au, or the like are formed by sputtering, deposition, or the like, a heater resistor 3, a common electrode, and a wiring electrode 4 for an individual electrode are formed by patterning in a photolithographic process, and, after that, a protective film 6 of SiO2, Ta2O5, SiAlON, Si3N4, Sic, or the like for inhibiting oxidation and for resisting wear of the heater resistor 3 is formed by sputtering, ion plating, or CVD to manufacture a thermal head.
In forming the protective film mentioned in the above, the protective film 6 has to be selectively formed in the heater resistor portion for the purpose of inhibiting oxidation and resisting wear, such that the protective film 6 does not remain at a portion 4a where the protective film is unnecessary such as a wire bonding portion to a driver IC for sending an image signal through the electrode to a heater resistor and the like. Several ways are conventionally known for selectively forming the protective film 6.
First, there is a way where physical masking is carried out. An example of this is shown in FIG. 2(a), where a metal mask 7 masks the substrate. With this method, since the metal mask 7 masks the substrate, not only can improvement of the positioning accuracy of the protective film 6 not be expected, but also its peeling off from the metal mask 7 is induced, leading also to decrease in the yield. Further, a space must be provided between the metal mask and the substrate such that the wiring electrode 4 is not damaged. There is a disadvantage that, here, the protective film 6 wraps around to the space between the metal mask 7 and the substrate, a protective film wraparound portion 6a is formed, and the protective film 6 remains even at the portion 4a where the protective film is unnecessary. In order to compensate for this point, in the step of designing, the protective film wraparound portion 6a is designed so as to be admitted, which is a factor that inhibits miniaturization of the substrate size, increase in the number of the thermal heads taken from one substrate, and the like.
Another way is to imbricate substrates. As shown in FIG. 2(b), since the substrates are imbricated, the wiring electrode 4 is damaged by contact. In order to prevent the wiring electrode 4 from being damaged, a space has to be provided between the substrates, which causes a disadvantage that the protective film 6 remains even at the portion 4a where the protective film is unnecessary. Further, for the purpose of imbricating the substrates, a wafer-like substrate has to be cut into long substrates. Cutting and imbricating the substrates takes time, causes increase in steps of the production process, and is a factor that increases the cost. In addition, since the substrates have to go through the production process in the cut state even at steps subsequent to the formation of the protective film 6, there is a disadvantage that the production touring is deteriorated.
Secondly, there is a way where the protective film 6 is chemically etched to selectively form the protective film 6. As the protective film 6 used in a thermal head, an inorganic ceramic film is used which is chemically and physically stable. Therefore, it is etched using a chemical of a hydrogen fluoride system. However, such a chemical has an extremely slow etching rate, which is a factor that lowers the productivity. This is true of not only etching using a chemical but also dry etching using a vapor phase method. In addition, etching using a chemical has a disadvantage that, since a metal is used as the wiring electrode 4, the etching selectivity to the protective film 6 can not be secured and even the wiring electrode 4 is etched. Therefore, this is not practical in the field of thermal heads.
As a way to solve these problems and to accommodate miniaturization of the substrate size and improvement in the productivity, selective formation of the protective film 6 using a masking agent, so-called lift-off, is known.
However, conventional selective formation of the protective film according to the lift-off is carried out using photoresist as the masking agent. In a method using photoresist, the protective film is formed at a high temperature in a high vacuum. In other words, the photoresist is exposed to the high temperature and the high vacuum. Since the photoresist is a resin, it can not withstand the conditions when the protective film is formed, and generates gas in a vacuum container. Such gas not only contaminates the inside of the vacuum container but also deteriorates the adhesion and the quality of the protective film, which may be a factor that decreases the reliability of the thermal head. Further, in case the masking agent is peeled off, since the resin is carbonized, i.e., burned out, it can not be peeled off, the masking agent remains on the wiring electrode at the portion where the protective film is unnecessary, wire bonding for connecting a driver IC for sending an image signal through the electrode to the heater resistor and the like can not be carried out, and the essential function of the thermal head is not carried out.
Further, a masking agent of a polyimide system which is more heat-resistant than such photoresist is also used. Though polyimide is heat-resistant, once it is cured, its peelability deteriorates extremely. At that time, although the amount is small the masking agent remains on the wiring electrode. If the masking agent remains, it becomes a factor that decreases the reliability in mounting and the productivity. For example, since the strength of the wire bonding for connection to a driver IC for sending an image signal through the electrode to the heater resistor and the like can not be secured, the wire bonding may be detached. To compulsorily peel it off, a polar solvent such as NMP for dissolving the polyimide has to be used. The use of such a polar solvent adversely affects the operator and the working environment. In addition, there is a problem that, since the consciousness of protecting the global environment has been raised recently, a strong chemical can not be used unconditionally.
Accordingly, an object of the present invention is, in order to solve the conventional problems mentioned in the above, to obtain a method of manufacturing a thermal head which can, by using inorganic paste as the masking agent, accommodate miniaturization of the substrate and an increased number of the thermal heads taken from one substrate, and which can selectively form a protective film with high positioning accuracy of the protective film, with high adhesion of the protective film, and with high reliability.
According to the present invention, in a method of manufacturing a thermal head having on an insulating substrate at least a heater resistor, a wiring electrode for supplying electric power to the heater resistor, and a protective film for covering the heater and the wiring electrode on the periphery thereof, at least the heater resistor and the wiring electrode for supplying electric power to the heater resistor are formed on the insulating substrate, a portion where the protective film is unnecessary of the wiring electrode where a driver IC for sending an image signal through the electrode to the heater resistor and the thermal head are connected by wire bonding is masked using inorganic paste, the protective film is formed over the whole surface, and then, the protective film of the portion where the protective film is unnecessary is peeled off together with the inorganic paste to selectively form the protective film on the heater and a heat generating portion of the wiring electrode on the periphery thereof
In a thermal head constituted as in the above, since the portion where the protective film is unnecessary is masked using the inorganic paste and the masking agent for forming the protective film contains no resin therein, the heat resistance is extremely high, and gas is not generated in a vacuum container at a high temperature in a high vacuum. Therefore, the inside of the vacuum container is not contaminated, and high adhesion of the film and high reliability of the film can be obtained. In addition, since its heat resistance is extremely high and it contains no resin component, there is no phenomenon such as carbonization and burnout, which facilitates its peeling off. Therefore, the masking agent does not remain on the wiring electrode, and thus, the strength of the wire bonding is improved. Further, since the masking agent can be used at an arbitrary position, the protective film can be formed selectively, and thus, the substrate size can be made smaller, the number of the thermal heads taken from one substrate increases, and the productivity is improved.